For example, in the shipping and aviation fields, the standardized communication system AIS is widely used for local communication between, for instance, moving vessels or between moving vessels and fixed installations, such as a port or a lighthouse. See for example “Recommendation ITU-R M.1371-4”, published in 2010 by the International Telecommunication Union.
According to this system, a set of digitally coded, predefined messages of different types can be used by, for example, a vessel for, among other things, informing about its current position and speed to recipients located in the local geographical neighbourhood of the vessel in question. The system is self-organizing in the sense that the participating transmitters divide their respective transmission over a predetermined periodically repeated time window of certain length, so that each transmitter uses a particular part of the time window for transmission.
AIS systems often have low security, in the sense that it is difficult to verify if the contents of the data being sent, such as the position of the transmitter, is accurate and consistent with the real situation. This means that transmitters wishing to participate are expected to follow the AIS standard and to transmit accurate information, and that only simple steps to verify the security in the received information are built into the system. For example, too large timing errors for transmitted signals are not allowed.
It would be desirable to provide an improved control ability of a receiver with respect to the information transmitted by a vessel, such as the vessel position, speed and heading. It would also be desirable to be able to detect whether a particular AIS signal is emitted by one and the same transmitter.
It is known to use directional antennas for measuring the direction to a radio transmitter, in order to thus verify the transmitter position. This is however costly and gives relatively poor precision.
It would be further desirable to provide a simple way for a vessel, whose local clock falls out of sync with the other transmitting vessels, to re-synchronize its local clock. Specifically, it would be desirable to provide a so-called secondary synchronization, which can be used as an alternative to the AIS standard supported synchronization process when the latter for some reason is not available.
It would also be desirable to provide a way for two or more transmitters/receivers to be able to jointly agree on a specific time definition, which is secret among the transmitter/receiver, without any other recipients also receiving information regarding such information.
These objectives should preferably be met using existing AIS standard equipment, in a cost-effective manner and with only a minimum of necessary modifications to existing AIS equipment.
The present invention solves the above described problems.